1. Field of the Invention
The present invention relates to a semiconductor device with an improved contact obtained by an etch selectivity difference between its polysilicon film and its oxide film and a method of making the same.
2. Description of the Prior Art
As the integration degree of semiconductor devices becomes high, the cell size is gradually reduced. All known techniques for forming active regions and element-isolation regions encounter a difficulty in obtaining sufficient areas for active regions, since the element-isolation oxide films form structures having a bird's beak shape with a certain size. This results in a difficulty in forming contacts at active regions.
Referring to FIGS. 1A to 1D, there is illustrated a conventional method of forming contacts.
In accordance with this method, first, on a semiconductor substrate 1a, element-isolation oxide films 1b having a bird's beak shape with a certain size are grown which define an active region and an insulating isolation region, as shown in FIG. 1A. In the active region, impurity ions are implanted so as to form a first diffusion region 2.
On the upper surface of the resulting entire structure including the active region and the insulating region are then deposited an insulating film 3 with a proper thickness and a photoresist 4, in this order, as shown in FIG. 1B. In FIG. 1B, the photoresist 4 is shown as having been patterned.
Using the pattern of the photoresist 4 as a mask, the insulating film 3 is then subjected to etching, so as to form a contact hole, as shown in FIG. 1C. At this time, each element-isolation oxide film 1b is also partially etched at its portion having the bird's beak shape.
As shown in FIG. 1D, a conductive material layer 5 is then deposited on the upper surface of the resulting entire structure such that the contact hole is filled with the conductive material layer 5. Since each element-isolation oxide film 1b is partially etched at its portion having the bird's beak shape, the conductive material layer 5 is deeply diffused at edges of the first diffusion region 2. As a result, a second diffusion region 6 which is unnecessary is formed.
Where the active region is wide, the element-isolation oxide films are not damaged by forming the contact hole at their portions having the bird's beak shape, as shown in FIG. 2, so that a normal contact can be obtained. In cases of highly integrated devices with narrow active regions as above-mentioned, however, the element-isolation oxide films are partially etched at their bird's beak portions each positioned at a boundary between the active region and the element-isolation oxide film, thereby causing the impurity junction structure at the etched region to be weak. As a result, there are problems of a weakness in junction breakdown voltage and an increase in junction leakage current.